chore: initialize NexaCore compiler workspace with basic frontend and CLI

Add initial project structure for NexaCore programming language compiler:
- Create Cargo workspace with 4 crates (cli, driver, frontend, runtime)
- Add lexer with indentation-based tokenization and keyword support
- Add parser for modules, functions, structs, and basic expressions
- Implement CLI with build command and placeholder subcommands
- Add driver crate to orchestrate compilation pipeline
- Include .gitignore for Rust build

examples/backend-api/main.nx

@@ -0,0 +1,30 @@
+use core.env
+use db.postgres.Pool
+use web.http.{App, Response}
+
+struct AppState:
+    pool: Pool
+
+async fn health(state: AppState) -> Response:
+    let version = env.get("APP_VERSION").or("dev")
+    let row = await state.pool.query_one<Map>(
+        "select now() as now"
+    )?
+
+    Response.json({
+        "status": "ok",
+        "version": version,
+        "database_time": row["now"]
+    })
+
+async fn main() -> Result<Void, AppError>:
+    let database_url = env.require("DATABASE_URL")?
+    let port = env.get("PORT").or("8080").to_int()?
+    let pool = Pool.connect(database_url, max: 16)?
+
+    let app = App.new()
+        .state(AppState { pool: pool })
+        .get("/health", health)
+
+    await app.listen("0.0.0.0", port)?
+